Fluid coupling

ABSTRACT

A fluid coupling transmits power from a drive side to a driven side by utilizing kinetic energy of fluid such as oil. The fluid coupling comprises a pump impeller ( 4 ) provided on a drive shaft ( 1 ), a turbine impeller ( 5 ) provided on a driven shaft ( 9 ), a housing ( 20 ) fixed to the pump impeller ( 4 ) and surrounding the turbine impeller, and a multiple disc clutch provided between a drive shaft side and a driven shaft side. The multiple disc clutch is operated to couple the drive shaft ( 1 ) and the driven shaft ( 9 ) mechanically so that the drive shaft and the driven shaft are rotated at the same rotational speed. Thus, the fluid coupling can improve a power transmitting efficiency because of no slip between the rotational speed of the drive shaft and the rotational speed of the driven shaft.

TECHNICAL FIELD

[0001] The present invention relates to a fluid coupling fortransmitting power from a drive side to a driven side by utilizingkinetic energy of fluid such as oil, and more particularly to a fluidcoupling which has a clutch mechanism for mechanically coupling a driveshaft coupled to a prime mover and a driven shaft coupled to a drivenmachine and can improve a power transmitting efficiency by equalizingthe rotational speed of the driven shaft to the rotational speed of thedrive shaft without any slip loss.

BACKGROUND ART

[0002] There has been known a fluid coupling which has a pump impellerconnected to a drive shaft and a turbine impeller connected to a drivenshaft and transmits power from the drive shaft to the driven shaft viafluid filled in a casing.

[0003] In case of performing a rotational speed control by means of afluid coupling, the rotational speed on a load side can be steplesslychanged from a minimum rotational speed to a maximum rotational speed byusing a scoop tube, or a minimum rotational speed or a maximumrotational speed on a load side can be obtained by feeding a workingfluid such as working oil to a coupling section of the fluid coupling orinterrupting the feeding of the working fluid to the coupling section.

[0004] In either case, when the driven shaft is rotated at a maximumrotational speed, the maximum rotational speed of the driven shaft isnormally lower than an input rotational speed of the prime mover such asa motor or an engine by about 2 to 3% due to slippage. This slippagecannot be avoid d when the rotational speed on the load side iscontrolled by the fluid coupling.

[0005] Specifically, when the coupling section of the fluid coupling isfilled with the working fluid up to 100%, the rotational speed of thedriven shaft reaches 97 to 98% of the rotational speed of the driveshaft, and hence power loss is no more than 2 to 3% due to slippage ofthe rotational speed. Therefore, special measures for reducing the powerloss caused by slippage have not been taken.

[0006] However, energy-saving has been highlighted in recent years, andslippage between the drive shaft and the driven shaft in the fluidcoupling is considered to be the subject of the energy-saving, and hencethere is a demand for eliminating such slippage generated when the fluidcoupling is operated at a high rotational speed, thus achievingenergy-saving.

DISCLOSURE OF INVENTION

[0007] It is therefore an object of the present invention to provide afluid coupling which has a clutch mechanism between a drive shaft sideand a driven shaft side and can improve a power transmitting efficiencyby equalizing the rotational speed of a driven shaft coupled to a drivenmachine to the rotational speed of a drive shaft coupled to a primemover without any slip loss.

[0008] In order to achieve the above object, according to the presentinvention, there is provided a fluid coupling comprising: a pumpimpeller provided on a drive shaft; a turbine impeller provided on adriven shaft; a housing fixed to the pump impeller and surrounding theturbine impeller; and a multiple disc clutch provided between a driveshaft side and a driven shaft side, the multiple disc clutch beingoperated to couple the drive shaft and the driven shaft mechanically sothat the drive shaft and the driven shaft are rotated at the samerotational speed.

[0009] According to the present invention, the clutch is providedbetween the drive shaft side and the driven shaft side in the fluidcoupling, and the clutch is operated to mechanically connect the driveshaft and the driven shaft after the rotational speed of the drivenshaft reaches a maximum, thereby mechanically coupling the drive shaftand the driven shaft to eliminate slippage between the drive shaft andthe driven shaft. The clutch comprises a multiple disc clutch which hasa simple structure and can be controlled easily, and the clutch can beconnected or disconnected by a fluid pressure such as an oil pressure.

[0010] In a preferred aspect of the present invention, the stroke of thepiston for pressing the clutch plates (friction plates) in the multipledisc clutch incorporated in the fluid coupling is limited.

[0011] With this arrangement, when the wear of the clutch plates(friction plates) exceeds a predetermined value, even if the pistonmoves in a full stroke, a friction force for transmitting power by thefriction plates cannot be exerted.

[0012] In a preferred aspect of the present invention, detectingapparatuses for detecting rotational speeds are provided on the driveshaft and the driven shaft. When the differential between the rotationalspeed of the drive shaft and the rotational speed of the driven shaft isgenerated due to slippage between the drive shaft and the driven shaftwhile the piston is pressed against the clutch plates, such differentialis detected by the detecting apparatuses. Thus, the state in which thewear of the clutch plates (friction plates) reaches a predeterminedvalue can be detected.

[0013] The above and other objects, features, and advantages of thepresent invention will be apparent from the following description whentaken in conjunction with the accompanying drawings which illustratespreferred embodiments of the present invention by way of example

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a schematic view showing a whole structure of a fluidcoupling having a multiple disc clutch according to a first embodimentof the present invention;

[0015]FIG. 2 is a cross-sectional view of a power transmitting sectionof the fluid coupling according to the first embodiment of the presentinvention;

[0016]FIG. 3 is a schematic view showing a whole structure of a fluidcoupling having a multiple disc clutch according to a second embodimentof the present invention;

[0017]FIG. 4 is a cross-sectional view of a power transmitting sectionof the fluid coupling according to the second embodiment of the presentinvention;

[0018]FIG. 5 is a cross-sectional view of the multiple disc clutchaccording to the second embodiment of the present invention;

[0019]FIG. 6 is a schematic view showing a whole structure of a fluidcoupling having a multiple disc clutch according to a third embodimentof the present invention; and

[0020]FIG. 7 is a cross-sectional view of a power transmitting sectionof the fluid coupling according to the third embodiment of the presentinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] A fluid coupling according to a first embodiment of the presentinvention will be described with reference to FIGS. 1 and 2.

[0022] The fluid coupling of the present invention has a multiple discclutch between a drive shaft side and a driven shaft side. When apressurized fluid such as pressurized oil is supplied to a hydrauliccylinder, the multiple disc clutch is pressed against each other, thusconnecting the clutch. In this case, as a method for producing apressurized fluid, there are a method in which a pressure produced by acentrifugal force applied to oil which rotates together with adrive-side rotating body is utilized, a method in which an oil pressureproduced by a hydraulic pump which is mechanically connected to a driveshaft is utilized, and a method in which an oil pressure produced by ahydraulic pump which is driven by an electric motor is utilized. Thefluid coupling shown in FIGS. 1 and 2 utilizes a pressure produced by acentrifugal force applied to oil which rotates together with adrive-side rotating body.

[0023] As shown in FIG. 1, a drive shaft 1 is rotatably supported byradial bearings 2 and thrust bearings 3. A pump impeller 4, afluid-coupling housing 20 and a clutch housing 13 are fixed to theforward end of the drive shaft 1. In the clutch housing 13, there areprovided a plurality of drive-side friction plates 7 and a clutch piston11. A cylinder chamber 10 is defined by the clutch housing 13 and theclutch piston 11.

[0024] A driven shaft 9 is disposed adjacent to the drive shaft 1, andis rotatably supported by radial bearings 2 and thrust bearings 3. Aturbine impeller (runner) 5 is fixed to the forward end of the drivenshaft 9, and is disposed so as to face the pump impeller 4. A pluralityof driven-side friction plates 12 are provided on the driven shaft 9,and these driven-side friction plates 12 are arranged so as to alternatewith the drive-side friction plates 7 in the clutch housing 13. Thedrive-side friction plates 7 and the driven-side friction plates 12which serve as clutch plates and are alternately arranged jointlyconstitute a multiple disc clutch. A stationary plate 22 is fixedlyprovided in the clutch housing 13 (see FIG. 2).

[0025] As shown in FIG. 1, a working fluid such as working oil is storedin a low r portion of a fluid-coupling casing 30. The working fluid inthe fluid-coupling casing 30 is pumped by a pump 16 mechanicallyconnected to the drive shaft 11 and is supplied under substantiallynon-pressure to the pump impeller 4 of the fluid coupling via afluid-coupling solenoid controlled valve 15.

[0026]FIG. 1 shows the state in which the working oil is pumped by thepump 16 by energizing the fluid-coupling solenoid controlled valve 15,and is supplied to the pump impeller 4. Kinetic energy is given to thesupplied oil by the pump impeller 4, and then such oil impinges upon theturbine impeller 5 to give energy to the turbine impeller 5, therebytransmitting power from the drive shaft 1 to the driven shaft 9. Theworking oil supplied to the fluid-coupling housing 20 which houses thepump impeller 4 and the turbine impeller 5 is discharged by acentrifugal force produced by rotation of the housing 20 from adischarge nozzle 6 provided in the outer peripheral portion of thefluid-coupling housing 20. The discharged working oil is returned to thelower portion of the fluid-coupling casing 30.

[0027] Because an amount of the working oil supplied through a supplypipe 17 is larger than an amount of the working oil discharged from thedischarge nozzle 6, the fluid-coupling housing 20 is rapidly filled withthe working oil, and the rotational speed of the driven shaft 9 isquickly increased and reaches a maximum within a short time of less thanfive seconds. When the driven shaft 9 is rotated at the maximumrotational speed, 2 to 3% of slippage is generated between the driveshaft 1 and the driven shaft 9. In this state, the working oil issupplied to the cylinder chamber 10 in the clutch housing 13 byenergizing a clutch solenoid controlled valve 14, and the working oilwhich has nt red the cylinder chamber 10 begins to rotate atsubstantially the same rotational speed as that of the clutch housing 13due to friction between the working oil and the inner surface of theclutch housing 13 and the surface of the clutch piston 11. Therefore, acentrifugal pressure is developed by a centrifugal force produced byrotation of the working oil in the cylinder chamber 10, and is appliedto the clutch piston 11 which is in turn pushed forward, thus pressingthe drive-side friction plates 7 against the driven-side friction plates12 in the multiple disc clutch. Because the drive-side friction plates 7and the driven-side friction plates 12 are arranged in an alternate row,the driven-side friction plates 12 are rotated at the same rotationalspeed as that of the drive-side friction plates 7. Thus, the drive shaft1 and the driven shaft 9 rotate at the same rotational speed without anyslip, and power is transmitted from the drive shaft 1 to the drivenshaft 9.

[0028] A discharge nozzle 8 is provided in the outer peripheral portionof the clutch housing 13, and the working oil in the cylinder chamber 10is always discharged little by little from the discharge nozzle 8 byrotation of the drive shaft 1. Therefore, when energization of theclutch solenoid controlled valve 14 is stopped, the working oil does notremain in the cylinder chamber 10. As a result, a force for pressing thedrive-side friction plates 7 by the clutch piston 11 disappears, and themechanical coupling of the multiple disc clutch is released.

[0029]FIG. 2 is a cross-sectional view of a power transmitting sectionof the fluid coupling having the multiple disc clutch therein accordingto the present invention. As shown in FIG. 2, the clutch housing 13 isfixed to the fluid-coupling housing 20 and a clutch hub 21 is fixed tothe turbine impeller 5. The drive-side friction plates 7 are held by theclutch housing 13 so as to be movable in an axial direction of the driveshaft 1. The driven-side friction plates 12 are held by the clutch hub21 so as to be movable in an axial direction of the driven shaft 9. Tobe more specific, the drive-side friction plates 7 are rotated togetherwith the clutch housing 13, but are movable in an axial direction of theclutch housing 13. The driven-side friction plate 12 are rotatedtogether with the clutch hub 21, but are movable in an axial directionof the clutch hub 21.

[0030] The clutch piston 11 is disposed in the clutch housing 13. Whenthe working oil is supplied to the cylinder chamber 10 in the clutchhousing 13, a centrifugal pressure is developed in the clutch chamber10, and hence the drive-side friction plates 7 are pressed against thedriven-side friction plates 12. Thus, the multiple disc clutch becomesin a coupling or connecting state. Therefore, the drive shaft 1 and thedriven shaft 9 are rotated at the same rotational speed without anyslip. Further, a radial bearing 31 is provided between the clutchhousing 13 and the clutch hub 21.

[0031] In the fluid coupling having the above structure, the working oilsupplied from an oil inlet 24 formed in the casing enters the pumpimpeller 4 fixed to the drive shaft 1, and energy is given to theworking oil by the pump impeller 4, and then the working oil enters theturbine impeller 5, thereby transmitting power from the drive shaft 1 tothe driven shaft 9. The discharge nozzle 6 is attached to thefluid-coupling housing 20. When supply of the working oil to the oilinlet 24 is stopped, the working oil which has resided in the pumpimpeller 4 and the turbine impeller 5 is discharged therefrom and doesnot remain therein, and thus the power transmission is not performed.

[0032] In the state in which the drive shaft 1 and the clutch housing 13are rotated, the working oil is supplied from an oil inlet 23 to thecylinder chamber 10, and the working oil in the cylinder chamber 10 isrotated to develop a pressure by a centrifugal force of the working oil.Thus, the clutch piston 11 is pushed forward by this pressure to pressthe drive-side friction plates 7 against the driven-side friction plates12. As a result, the drive shaft 1 and the driven shaft 9 aremechanically coupled, and the drive shaft 1 and the driven shaft 9 arerotated at the same speed without any slip, and power is transmittedfrom the drive shaft 1 to the driven shaft 9.

[0033] Because the working oil in the cylinder chamber 10 is alwaysdischarged from the discharge nozzle 8, when supply of the working oilto the oil inlet 23 is stopped, the mechanical coupling of the driveshaft 1 and the driven shaft 9 is released.

[0034]FIGS. 3 through 5 are views showing a fluid coupling according tothe second embodiment of the present invention. FIG. 3 is a schematicview of the fluid coupling having a multiple disc clutch therein, FIG. 4is a cross-sectional view of a power transmitting section of the fluidcoupling, and FIG. 5 is a cross-sectional view of the multiple discclutch. In the fluid coupling shown in FIGS. 3 through 5, there areprovided a device for limiting a stroke of the piston for pressing theclutch plates of the multiple disc clutch in the fluid coupling, and adevice for detecting an amount of wear of the friction plates of themultiple disc clutch.

[0035] In FIGS. 3 through 5, the parts or elements which are identicalto those shown in FIGS. 1 and 2 are denoted by the same referencenumerals.

[0036] The structure of the fluid coupling shown in FIG. 3 is almost thesame as the fluid coupling shown in FIG. 1. As shown in FIG. 3, adetecting apparatus 32 for detecting a rotational speed of a drive shaftis attached to the drive shaft 1, and a detecting apparatus 33 fordetecting a rotational speed of a driven shaft is attach d to the drivenshaft 9.

[0037] If the differential between the rotational speed of the driveshaft 1 and the rotational speed of the driven shaft 9 is generated,then the detecting apparatus s 32 and 33 issue a warning to notify thewear of the friction plates.

[0038] A paper based frictional material having a large coefficient offriction and a thickness of about 1 to 2 mm is attached to the surfacesof the drive-side friction plate 7 and the driven-side friction plate12, or sintered alloy is deposited on the surfaces of the drive-sidefriction plate 7 and the driven-side friction plate 12.

[0039] Further, as shown in FIGS. 4 and 5, a stroke adjustment washer 39and a coned disk spring 40 are interposed between the clutch housing 13and the clutch piston 11. Other structure of the fluid coupling shown inFIGS. 3 through 5 is the same as that of the fluid coupling shown inFIGS. 1 and 2.

[0040]FIG. 4 is a view showing the state in which the clutch solenoidcontrolled valve 14 is energized and the working oil is supplied to thecylinder chamber 10 to press the friction plates against each other bythe clutch piston 11, thereby connecting the multiple disc clutchmechanically. Specifically, in the state shown in FIG. 4, the clutchsolenoid controlled valve 14 is energized and the drive-side frictionplates 7 are pressed against the driven-side friction plates 12 by theclutch piston 11 to mechanically connect the multiple disc clutch, andhence the drive shaft 1 and the driven shaft 9 are rotated at the samerotational speed, and power is transmitted from the drive shaft 1 to thedriven shaft 9. The discharge nozzle 8 is provided in the outerperipheral portion of the clutch housing 13 which constitutes the outercircumference of the cylinder chamber 10, and the working oil in thecylinder chamber 10 is always discharged little by little to theexterior by rotation of the drive shaft 1. Therefore, when energizationof the clutch solenoid controlled valve 14 is stopped, the working fluidis discharged from the cylinder chamber 10 and does not remain therein,and the force for pressing the drive-side friction plates 7 against thedriven-side friction plates 12 by the clutch piston 11 disappears.Therefore, the clutch piston 11 is pushed back by the urging force ofthe coned disk spring 40 located between the stroke adjustment washer 39and the clutch piston 11, and hence the mechanical coupling of themultiple disc clutch is released.

[0041]FIG. 5 is a cross-sectional view of the multiple disc clutchsection, and shows the state in which the multiple disc clutch isreleased. Specifically, in the state shown in FIG. 5, energization ofthe clutch solenoid controlled valve 14 is stopped, the clutch piston 11having a predetermined set stroke (S), e.g. 5 mm is pushed back by theurging force of the coned disk spring 40, and the multiple disc clutchis disconnected.

[0042] According to the present embodiment, as shown in FIGS. 3 through5, the stroke of the clutch piston 11 for pressing the clutch plates(friction plates) in the multi-dick clutch incorporated in the fluidcoupling is limited by the stroke adjustment washer 39. Therefore, whenthe amount of wear of the clutch plates (friction plates) exceeds apredetermined value, even if the clutch piston 11 moves in a fullstroke, the friction force for transmitting power by the friction platescannot be exerted.

[0043] Further, the detecting apparatus 32 for detecting a rotationalspeed of a drive shaft is provided on the drive shaft 1, and thedetecting apparatus 33 for detecting a rotational speed of a drivenshaft is provided on the driven shaft 9. Therefore, when thedifferential between the rotational speed of the drive shaft 1 and therotational speed of the driven shaft 9 is generated due to the slippagebetween the drive shaft 1 and the driven shaft 9, the detectingapparatuses 32 and 33 detect the wear of the clutch plates (frictionplates) which reaches a predetermined value, and issues a warning. Thedetecting apparatus s 32 and 33 may b combined to construct a singledetecting apparatus for detecting the differential between therotational speed of the drive shaft 1 and the rotational speed of thedriven shaft 9.

[0044]FIGS. 6 and 7 are views showing a fluid coupling according to thethird embodiment of the present invention. FIG. 6 is a schematic view ofthe fluid coupling having a multiple disc clutch therein, and FIG. 7 isa cross-sectional view of a power transmitting section of the fluidcoupling.

[0045] In the embodiment shown in FIGS. 6 and 7, at a certain locationof the supply pipe 17, there is provided a bypass passage Bp, having anorifice 41 for setting a minimum amount of the working oil, which isbypassed from a main passage which has the fluid-coupling solenoidcontrolled valve 15 (see FIG. 6). Further, at the location inside of thedischarge nozzle 6, there is provided a dam 42 which projects radiallyinwardly from the inner wall surface of the fluid-coupling housing 20(see FIG. 7). A minimum rotational speed of the fluid coupling can beset by the dam 42. Specifically, the minimum rotational speed of thefluid coupling can be adjusted by changing the height h of the dam 42from the inner wall surface of the fluid-coupling housing 20, and hencethe dam 42 constitutes a minimum rotational speed setting dam.

[0046] In the embodiment shown in FIGS. 6 and 7, the fluid-couplingsolenoid controlled valve 15 is closed during the minimum rotationalspeed operation, and the amount of the working oil determined by theorifice 41 is supplied to the working oil chamber comprised by the pumpimpeller 4, the turbine impeller (runner) 5 and the fluid-couplinghousing 20, and then the supplied working oil overflows the dam 42.Thereafter, the working oil is discharged from the discharge nozzle 6and is returned to the lower portion of the fluid-coupling casing 30. Atthis time, the amount of the working oil supplied through the orifice 41to the working oil chamber is smaller than the amount of the working oildischarged through the discharge nozzle 6 from the working oil chamber,and hence the driven shaft 9 is rotated at the minimum rotational speeddetermined by the amount of the working oil, within the working oilchamber, which is set by the height h of the dam 42.

[0047] When the fluid-coupling solenoid controlled valve 15 is energizedand opened, because the amount of the working oil supplied through thefluid-coupling solenoid controlled valve 15 and the orifice 41 to theworking oil chamber is larger than the amount of the working oil whichis discharged through the discharge nozzle 6 from the working oilchamber, the working oil chamber is rapidly filled with the working oil.Thus, the rotational speed of the driven shaft 9 is rapidly increasedand reaches a maximum within a short time of less than five seconds.This maximum rotational speed of the driven shaft 9 corresponds to thespeed subtracted from the rotational speed of the drive shaft 1 by theamount of slippage. In this state, the clutch solenoid controlled valve14 is energized, and the clutch is connected. As a result, the driveshaft 1 and the driven shaft 9 are mechanically coupled, and the driveshaft 1 and the driven shaft 9 are rotated at the same speed without anyslip.

[0048] The rapid acceleration-type fluid coupling having the minimum oilamount setting orifice and the minimum rotational speed setting dam isparticularly suitable for the usage in which the response time from theminimum rotational speed to the maximum rotational speed and theresponse time from the maximum rotational speed to the minimumrotational speed are required to be extremely short. That is, this rapidacceleration-type fluid coupling is suitable for the rotational speedcontrol of a driven machine which repeats frequently load running andnon-load running, for example, a pump for a descaling apparatus in a hotrolling system.

[0049] As shown in FIGS. 6 and 7, by incorporating the clutch mechanismof the present invention into the rapid acceleration-type fluidcoupling, after the driven shaft reaches a maximum rotational speed, theclutch is connected to eliminate the slippage between the drive shaftand the driven shaft, and hence great energy-saving can be achieved.

[0050] In the embodiments shown in FIGS. 1 through 7, as a device forpressing the clutch plates in the multiple disc clutch, an oil pressuregenerated by a centrifugal force caused by rotation of the drive-siderotating body is applied to the clutch plate pressing chamber (cylinderchamber 10) to press the clutch plates against each other, therebymechanically coupling the drive shaft 1 and the driven shaft 9. However,the following devices may be employed for pressing the clutch plates.

[0051] (1) As a device for pressing the clutch plates in the multipledisc clutch, an oil pressure developed by a hydraulic pump driven by thedrive shaft is applied to the clutch plate pressing chamber (cylinderchamber 10) to press the clutch plates, thereby mechanically couplingthe drive shaft and the driven shaft.

[0052] (2) As a device for pressing the clutch plates in the multipledisc clutch, an oil pressure developed by a hydraulic pump driven by anelectric motor is applied to the clutch plate pressing chamber (cylinderchamber 10) to press the clutch plates, thereby mechanically couplingthe drive shaft and the driven shaft.

[0053] As described above, according to the fluid coupling of thepresent invention, although a large-sized rotating machine can bestarted or stopped easily as with the conventional fluid coupling, afterthe large-sized rotating machine is started and then reaches apredetermined rotational speed, the drive shaft and the driven shaft inthe fluid coupling can be mechanically connected, thus eliminating anyslip and achieving energy-saving.

[0054] Further, according to the present invention, even if the wear ofthe paper based friction material or the sintered alloy applied to thesurfaces of the drive-side friction plate and the driven-side frictionplate in the multiple disc clutch incorporated in the fluid coupling isgenerated, the friction plates can be replaced with new friction platesbefore ground metal of the friction plates is exposed. This is becausethe slippage between the drive shaft and the driven shaft in the fluidcoupling can be detected. The friction plates whose friction materialhas been worn away can be repaired and reutilized.

[0055] Although certain preferred embodiments of the present inventionhave been shown and described in detail, it should be understood thatvarious changes and modifications may be made therein without departingfrom the scope of the appended claims.

INDUSTRIAL APPLICABILITY

[0056] The present invention is applicable to a fluid coupling fortransmitting power from a drive side such as prime mover to a drivenside such as a driven machine by utilizing kinetic energy of fluid suchas oil, and more particularly to a fluid coupling which is used for apump for a descaling apparatus in a hot rolling system.

1. A fluid coupling comprising: a pump impeller provided on a driveshaft; a turbine impeller provided on a driven shaft; a housing fixed tosaid pump impeller and surrounding said turbine impeller; and a multipledisc clutch provided between a drive shaft side and a driven shaft side,said multiple disc clutch being operated to couple said drive shaft andsaid driven shaft mechanically so that said drive shaft and said drivenshaft are rotated at the same rotational speed.
 2. A fluid couplingaccording to claim 1, further comprising a pressing device for pressingclutch plates in said multiple disc clutch, said pressing devicecomprising a clutch plate pressing chamber to which a fluid pressuregenerated by a centrifugal force developed by rotation of a drive-siderotating body is applied.
 3. A fluid coupling according to claim 1,further comprising a pressing device for pressing clutch plates in saidmultiple disc clutch, said pressing device comprising a clutch platepressing chamber to which a fluid pressure generated by a hydraulic pumpdriven by said drive shaft is applied.
 4. A fluid coupling according toclaim 1, further comprising a pressing device for pressing clutch platesin said multiple disc clutch, said pressing device comprising a clutchplate pressing chamber to which a fluid pressure generated by ahydraulic pump driven by a motor is applied.
 5. A fluid couplingaccording to claim 1, further comprising: a pressing device for pressingclutch plates in said multiple disc clutch; said pressing devicecomprising: a clutch plate pressing chamber to which a fluid pressure isapplied; a piston provided in said clutch plate pressing chamber forpressing said clutch plates; and a device for limiting a stroke of saidpiston.
 6. A fluid coupling according to claim 5, further comprising astroke adjustment device for adjusting said stroke of said piston.
 7. Afluid coupling according to claim 1, further comprising a detectingapparatus for detecting a differential between a rotational speed ofsaid drive shaft and a rotational speed of said driven shaft.
 8. A fluidcoupling according to claim 7, wherein a warning is issued when saiddetecting apparatus detects said differential. multiple disc clutchbeing operated to couple said drive shaft and said driven shaftmechanically so that said drive shaft and said driven shaft are rotatedat the same rotational speed; a pressing device for pressing clutchplates in said multiple disc clutch; said pressing device comprising: aclutch plate pressing chamber to which a fluid pressure is applied; apiston provided in said clutch plate pressing chamber for pressing saidclutch plates; and a device for limiting a stroke of said piston;wherein when the amount of wear of said clutch plates exceeds apredetermined value, the stroke of said piston is limited by said deviceso that the frictional force for transmitting power by said clutchplates cannot be exerted to generate a differential between a rotationalspeed of said drive shaft and a rotational speed of said driven shaft.